The general principles of moving coil electrodynamic loudspeakers are well understood. Central to the ability of a transducer to generate sound is the concept of volume displacement. The volume displacement of a transducer with a single diaphragm is equal to the effective surface area of the diaphragm multiplied by the excursion capability of that diaphragm. The greater the volume displacement of a transducer, the greater its potential for generating sound. The need for large volume displacement is especially pronounced at low frequencies. The traditional methods for achieving greater volume displacement in a transducer are to increase the surface area of the diaphragm, to increase the excursion capability of the diaphragm, or both.
Traditional transducers that are used to produce significant low frequency energy incorporate a single diaphragm with a large surface area and use motors and housings that provide for adequate excursion of the diaphragm. This leads to certain minimum dimension requirements for the diaphragm of a loudspeaker, which in turn imposes minimum dimension requirements on the loudspeaker enclosure. It is very difficult to use traditional transducers with good low-frequency response in applications such as flat-panel television and computer monitors. In these applications, the current solution is to use a separate subwoofer box to reproduce low frequency sound, resulting in added cost and inconvenience. The same holds true of automotive sound system applications, where designers struggle to find a place to hide the subwoofer in the car, which is usually in the trunk or under the seats.